Can somebody help us with this issue?

We are a UK company, always have been and have been selling on Amazon for many years. Recently we deregistered for VAT and let amazon know about this. We have provided masses of information about the company and the owner to Amazon confirming that we are a UK company. However, they believe we are not in the UK and will now calculate the VAT we owe? We have been paying VAT in the UK until the point we deregistered and can't understand why they're doing this!